Bash Arguments

Command line arguments are values typed after the script name. They let one script handle many different jobs without any code change. The command cp file1.txt backup/ works the same way: cp is the program, and the two words after it are arguments.

How Arguments Reach a Script

Bash stores each argument in a numbered variable. The number shows the position.

  ./greet.sh   Asha   Pune   Blue
       |        |      |      |
       |        |      |      +--> $3
       |        |      +---------> $2
       |        +----------------> $1
       +-------------------------> $0 (script name)

Special Argument Variables

Variable	Meaning
$0	Name of the script
$1 to $9	First to ninth argument
${10}	Tenth argument and beyond need braces
$#	Count of arguments
$@	All arguments as separate words
$*	All arguments as one single string
$?	Exit status of the last command

Read Arguments in a Script

Create a file named greet.sh:

#!/bin/bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Total arguments: $#"

Run it with two values:

./greet.sh Asha Pune

Output:

Script name: ./greet.sh
First argument: Asha
Second argument: Pune
Total arguments: 2

Loop Over Every Argument

The $@ variable suits any number of values:

#!/bin/bash
for item in "$@"
do
  echo "Processing: $item"
done

Run ./list.sh apple "red grape" mango and the loop treats red grape as one item, because the quotation marks keep the words together.

Check for Missing Arguments

A good script warns the user when a required value is absent:

#!/bin/bash
if [ $# -lt 1 ]; then
  echo "Usage: $0 filename"
  exit 1
fi
echo "Reading $1"

The exit 1 line stops the script and reports a failure.

Use the shift Command

The shift command drops the first argument and moves the others one place left. Scripts use it to walk through a long list.

#!/bin/bash
while [ $# -gt 0 ]
do
  echo "Current: $1"
  shift
done

Run ./walk.sh a b c to get a, b, and c on separate lines.

Options With Flags

Many tools accept flags such as -n or -v. The getopts command reads them in a clean way.

#!/bin/bash
while getopts "n:v" option
do
  case $option in
    n) name=$OPTARG ;;
    v) verbose=true ;;
    *) echo "Usage: $0 -n name [-v]"; exit 1 ;;
  esac
done

echo "Name: $name"
if [ "$verbose" = true ]; then
  echo "Verbose mode is on"
fi

Run the script like this:

./opts.sh -n Asha -v

Output:

Name: Asha
Verbose mode is on

The letter n: in the pattern needs a value after it. The letter v works as a simple on/off switch.

Practical Example: Add Two Numbers

#!/bin/bash
if [ $# -ne 2 ]; then
  echo "Usage: $0 number1 number2"
  exit 1
fi
sum=$(( $1 + $2 ))
echo "Sum is $sum"

Run ./add.sh 15 27 and the screen shows Sum is 42.

Set Default Values

name=${1:-Guest}
echo "Hello, $name"

The script prints Hello, Guest when no argument arrives.

Key Takeaways

• Use $1, $2, and so on to read arguments by position.
• $# gives the count and $@ gives the full list.
• Wrap "$@" in quotation marks to protect values that contain spaces.
• shift removes the first argument from the list.
• getopts handles flags like -n and -v.
• Check the argument count before using any value.